1. Field of the Invention
This invention relates to a method for forming a gate, and more particularly to a method for forming a poly metal gate having a stack structure of a refractorymetal film, a barrier film and a polysilicon film.
2. Description of the Related Art
As well known, a polysilicon is widely used as an electrode material and an interconnection material in semiconductor devices. As integration degree and speed of devices are increased, the signal transfer delay arises from the resistances of an electrode and an interconnection. The delay is suppressed by lowering the resistances of an electrode and an interconnection. For example, in case of a gate electrode in a MOS transistor, the delay is suppressed by using a two-layered polycide gate of a metal silicide film and a polysilicon film.
However, if the gate electrode has a length of below 0.25 .mu.m, the use of a gate electrode having resistance lower than the polycide gate is demanded. Recently, a poly metal gate having a stack structure of a refractory metal film, a barrier metal film and a polysilicon film is remarked. For example, if a tungsten film is used as a refractory metal film, the RC delay time can be remarkably reduced because the resistivity of the tungsten is smaller than that of the tungsten silicide Wsi.sub.x by approximately an order of magnitude. On the other hand, tungsten is a material that reacts with polysilicon in the heat treatment of about 600.degree. C. But the barrier metal disposed between the tungsten and polysilicon prevents the reaction between them.
Generally, in the LSI manufacturing process, it is necessary to carry out reoxidation process so as to improve reliability of an oxide such as a gate oxide, following the formation of a gate electrode. However, if the reoxidation process is adapted to a poly metal gate using tungsten as a refractory metal, tungsten is easily oxidized so that the delay of the poly metal gate using tungsten as a refractory metal becomes larger. That is, tungsten is oxidized at a temperature of about 400.degree. C. and the tungsten oxide is an insulating material. Tungsten causes a cubical expansion in oxidation and an exposed portion of the tungsten film is oxidized during the reoxidation process so that the delay of the poly metal gate using tungsten as a refractory metal becomes more increased.
On the other hand, in case where a portion of Si is exposed as well as a portion of refractory metal is exposed like the poly metal gate, a selective oxidation method which selectively oxidizes only the exposed Si without oxidizing the exposed refractory metal, has bee used. When the poly metal gate using tungsten as a refractory metal is formed, the selective oxidation technique solves the problem that the exposed tungsten film as a refractory film is oxidized in the reoxidation process. However, the selective oxidation technique is performed in a high temperature and has a thermal budget problem so that it is substantially impractical. Recently, the technique for forming a capping layer for preventing oxidation in the exposed portion of the refractory metal such as tungsten is proposed. A nitride film is used as the capping layer for preventing oxidation. The nitride film is deposited over a substrate to cover the poly metal gate, before the reoxidation. Then the nitride film is etched by a reactive ion etching method to remain in the sidewalls of the poly metal gate including the exposed portion of the refractory metal.
However, the technique using the capping layer has a disadvantage in that the exposed portion of the tungsten film is oxidized as usual in the following reoxidation in spite of the formation of the nitride film for oxidation prevention. It is because the nitride film for oxidation prevention is damaged in the previous etching process. Therefore, because the reoxidation is carried out under the state that the nitride film is damaged, the oxidizing gas that is penetrated through the damaged portion of the nitride film oxidizes the exposed portion of the tungsten film. Besides, if the exposed portion of the tungsten film is oxidized, the cohesive force between the tungsten film and the nitride film becomes poorer and the lifting of the nitride film is caused.